4.2.6 Order of network evaluation
Within a program organization unit written in LD, networks shall be evaluated in top to bottom order as
they appear in the ladder diagram, except as this order is modified by the execution control elements
defined in 4.1.4.
Table 61 - Contacts
a
Static contacts
No.
Symbol
Description
1
2
***
--| |--
or
***
--! !--
Normally open contact
The state of the left link is copied to the right link if the
state of the associated Boolean variable (indicated by
"***") is ON. Otherwise, the state of the right link is
OFF.
3
4
***
--|/|--
or
***
--!/!--
Normally closed contact
The state of the left link is copied to the right link if the
state of the associated Boolean variable is OFF.
Otherwise, the state of the right link is OFF.
Transition-sensing contacts
5
6
***
--|P|--
or
***
--!P!--
Positive transition-sensing contact
The state of the right link is ON from one evaluation of
this element to the next when a transition of the
associated variable from OFF to ON is sensed at the
same time that the state of the left link is ON. The state
of the right link shall be OFF at all other times.
7
8
***
--|N|--
or
***
--!N!--
Negative transition-sensing contact
The state of the right link is ON from one evaluation of
this element to the next when a transition of the
associated variable from ON to OFF is sensed at the
same time that the state of the left link is ON. The state
of the right link shall be OFF at all other times.
a
As specified in 2.1.1, the exclamation mark “!” shall be used when a national
character set does not support the vertical bar “|”.
Copyright International Electrotechnical Commission
Provided by IHS under license with IEC
Not for Resale
No reproduction or networking permitted without license from IHS
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– 142 –
61131-3
IEC:2003(E)
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